1. Technical Field
The present invention relates to an audio reproduction device for use with a video reproduction device, such as a television receiver, and to an audio-video reproduction system including the combination of an audio reproduction device and a video reproduction device and, particularly, relates to an audio reproduction device and an audio-video reproduction system which reproduce sounds in a stereo system or in a multi-channel system.
2. Background Art
In recent years, there have been increasingly users who enjoy so-called home theater in households, using audio reproduction devices constituted by amplifiers and speakers with excellent sound quality, in combination with thin-type large screen television receivers for reproducing clear images. Generally, television receivers have been used by being placed on furniture, such as television racks and tables with smaller heights (hereinafter, abbreviated as “low boards”). Therefore, the height of the center of a television screen is close to the height of a viewer's eye level. However, in many audio reproduction devices, the speakers for generating sounds are placed below the position of the center of the television screen.
There are many devices having placement states similar thereto, among audio reproduction devices incorporated in television receivers. For example, there are cases where, in order to cause a television receiver to have a smallest possible lateral width, the speakers are mounted thereto below the television screen.
In any of the aforementioned cases, sounds are heard from a different position from that of images, which has induced the problem that images and audio images are not coincident with each other and, therefore, the viewers have uncomfortable feelings. For example, in cases where the face of a speaking person appears on a television screen at the center thereof while the voices of this person are heard from below the television screen, the viewers have significantly uncomfortable feelings. Accordingly, by enabling ascent of audio images of reproduced sounds from speakers existing at lower positions, it is possible to overcome the aforementioned problem.
In order to overcome the aforementioned problem, there has been suggested, in JP-A No. 5-219597, an audio reproduction device which aims at ascending audio images of reproduced sounds from speakers placed below a television screen. This conventional audio reproduction device will be described, with reference to FIG. 21, FIG. 22, and FIG. 23.
FIG. 21 is a block diagram illustrating the structure of the conventional audio reproduction device disclosed in JP-A No. 5-219597. In the conventional audio reproduction device illustrated in FIG. 21, signals from an input signal source 44 are corrected in phase by a phase correction circuit 48, and the corrected signals are inputted to a mixing circuit 50 at its one terminal. Further, signal components centered at 7 kHz are extracted from the input signal source 44 by a filter circuit 43 and are amplified by a boost circuit 47 and, thereafter, these signals are controlled by a switch circuit 49b which are operated by signals from a random signal generator 49a and, then, are inputted to the other terminal of the mixing circuit 50. The mixing circuit 50 mixes the signals corrected in phase by the phase correction circuit 48 with the signals controlled by the switch circuit 49b and, further, outputs these signals resulted from the mixing to a speaker 41. The speaker 41 reproduces the signals resulted from the mixing.
Further, in cases where the boost circuit 47 and the like induce only extremely smaller phase shifts, it is possible to eliminate the phase correction circuit 48. The switch circuit 49b is provided in order to continuously change over signals, since ascent of audio images is felt most strongly at the time of changeover to boosted signals.
FIG. 22 is a sound-pressure frequency characteristic view illustrating ear's sensitivity to sounds reaching the viewer's ears from sound sources existing at a position at 30 degrees upwardly (+30 degrees) in the median plane and at a position at 30 degrees downwardly (−30 degrees) in the median plane, on the assumption that a state of 0 degree in the median plane is the case where the sound source is placed just in front of the viewer, namely the radiation axis (the center axis) of the speaker is horizontal and, also, the viewer's ears exist on this center axis. The term “30 degrees upwardly (+30 degrees) in the median plane” means the case where the sound source is placed on a line forming an angle of dip of 30 degrees in a vertically upward direction with respect to the horizontal line in the median plane. Further, the term “30 degrees downwardly (−30 degrees) in median plane” means the case where the sound source is placed on a lime forming an angle of dip of 30 degrees in a vertically downward direction with respect to the horizontal line in the median plane.
FIG. 23 is a sound-pressure frequency characteristic view resulted from the addition of a sound-pressure frequency characteristic indicating the ear sensitivity at 30 degrees upwardly in the median plane to the opposite characteristic resulted from upwardly and downwardly reversing, about an axis at 0 dB, the sound-pressure frequency characteristic indicating the ear sensitivity at 30 degrees downwardly (−30 degrees) in the median plane, which is illustrated in FIG. 22. In the sound-pressure frequency characteristic illustrated in FIG. 21, there is formed a single peak in a frequency range centered at 7 kHz.
In the conventional audio reproduction device illustrated in FIG. 21, based on the sound-pressure frequency characteristic illustrated in FIG. 23, the filter circuit 43 is caused to extract a frequency range centered at a frequency of 7 kHz, further, the extracted frequency range centered at a frequency of 7 kHz is boosted by the boost circuit 47, and the boosted frequency range is reproduced by the speaker 41. Further, in recent years, sound-pressure frequency characteristics indicative of ear sensitivities have been referred to as “head-related transfer functions” and, therefore, in the following description, the term “head-related transfer function” will be employed.
When the sound source exists at a position at 30 degrees downwardly (−30 degrees) in the median plane, signals having the opposite characteristics are formed by upwardly and downwardly reversing, about the axis at 0 dB, the head-related transfer function at 30 degrees downwardly in the median plane, and these signals are reproduced by the speaker 41, in order to correct the sounds, such that the sounds can be heard in a direction at 0 degree in the median plane (just in front of the viewer). Further, the head-related transfer function in a direction at 30 degrees upwardly (+30 degrees) in the median plane can be superimposed on the opposite characteristics, so that the sounds can be corrected such that the sounds are heard in the direction at 30 degrees upwardly (+30 degrees) in the median plane. Accordingly, it has been considered that, even when the speaker exists below the viewer's ears, it is possible to perform correction such that sounds can be heard as if the speakers existed above the viewer's ears.
Further, “J. BLAUERT “Sound Localization in the Median Plane”, ACUSTICA Vol. 22, 205-213, 1969/70 (hereinafter, abbreviated as BLAUERT)” describes results of experiments indicating that sounds at about 8 kHz can cause humans to have feelings as if the sounds were heard from above their heads. Based on such a literature, there have been provided audio reproduction devices capable of ascending audio images from speakers for making audio images and images coincident with each other, by emphasizing sounds in a frequency range of 7 to 8 kHz, even when the speakers are placed below the center of the television screen.    Patent Literature 1: Japanese Unexamined Patent Publication JP-A No. 5-219597    Non-Patent Literature 1: J. BLAUERT “Sound Localization in the Median Plane”, ACUSTICA Vol. 22, 205-213, 1969/70